1. Field of the Invention
The present invention relates generally to memory systems, and in particular to memory systems having programmable control parameters which control the operation of the memory and which can be reprogrammed after the memory system has been fabricated.
2. Background Art
Integrated circuit memory systems have been developed having very large storage capacities. Although attempts have been made to carefully control the fabrication of these memory systems so as to increase the yield, there invariably will be differences in memory system characteristics even for memory systems that utilize the same design. These differences in characteristics are attributable to many factors but the majority of differences are due to processing variations. Thus, memory systems taken from one semiconductor wafer may differ significantly from those taken from other wafers.
It is usually not possible to fully characterize a memory system until it is completely fabricated. At that point, it is not possible to modify the memory system to take into account the characteristics of the memory. By way of example, in a flash memory system it is possible to electrically program, read and erase the flash cells. The logic state of the cell is set by either programming or erasing the cell, with programming and erasing functioning to alter the threshold voltage of the cell. When the cell is read, the threshold voltage of the cell is determined in order to establish the logic state of the cell. However, the characteristics of the flash cell, such as the exact manner in which the flash cells respond to programming pulses or erase pulses, are not easily ascertained. The threshold voltage of the cell of one memory system may change one amount after a single programming pulse is applied and a cell of a another supposedly identical memory system may change a different amount after the same programming pulse is applied to it.
In order to accommodate these variations in memory system characteristics, it is typically necessary to design the system assuming worst case conditions. In that event, the overall performance of the system will almost by necessity be reduced.
Also, many processor systems which operate with an associated memory require a particular memory configuration to operate properly. By way of example, some systems require a word length of eight bits and some require sixteen bits. There are conventional memory systems available which permit the end user to control the word size to some degree. However, this somewhat increases the complexity imposed upon the end user of the memory since the end user must provide the necessary signals to the memory for controlling the word length. As a further example, most processor systems look to a certain portion of a memory for boot data at power on. Such boot data is necessary for the processor to function in system. The processor will be implemented to expect the boot data to be at a specific memory address. Some processors expect the boot data to be at the memory low addresses (bottom boot) and some processors expect the boot data to be at the memory high addresses (top boot).
In order to provide capabilities for different types of processor systems, it is possible to produce a different memory system for each application. However, it is always desirable to limit the number of different memory types which must be manufactured.
It would be desirable to have a memory system where the system can be fully characterized after fabrication and wherein certain operating parameters can then be permanently adjusted so as to provide a memory system that is optimized to take into account the particular characteristics of the system. It would also be desirable to provide the capability for modifying the configuration of a memory system after fabrication so as to reduce the number of different types of memories which must be fabricated. The present invention provides these features and other improved features as will become apparent to those skilled in the art upon a reading of the Detailed Description of the Invention together with the drawings.